1. Field of the Invention
The present invention relates to the regulation of the flow of particulate material, especially fine powder used in certain metallurgical processes. A special purpose of the invention is the regulation of the flow of a fine stream of metal powder to the focal point of a laser beam which is part of a process for building metal articles. Such processes, variously referred to as a xe2x80x9cconsolidation processxe2x80x9d, xe2x80x9claser sinteringxe2x80x9d, xe2x80x9cprecision laser weldingxe2x80x9d or xe2x80x9clayered depositionxe2x80x9d, are used for making turbine blades and other parts of special shape which are difficult to make by other means.
2. Prior Art
While many valves and flow regulators have been designed for controlling flow of particulate materials, none are known to Applicants which can accurately control flow when small volumes of fine powder such as are required for the consolidation type processes referred to above. These need flow rates of less than 7 cubic centimeters per minute, and often much less, down to a fraction of 1 cc per minute, and the flow rate needs to be changed quickly and smoothly. Commercial devices tend to have flow rates more than ten times what is required for the consolidation process.
Most known systems for powder flow control meter the powder from a hopper in an attempt to get it to flow in a continuous stream at a required rate. Hitherto, in the consolidation process, two commercial systems have been used. In one case the powder is metered by a rotating auger, then transported by pressurized inert gas through tubing to a nozzle that injects the powder into the molten pool. In the second, an aspirator system is used to pull the powder into a gas stream through a venturi. Generally, these and other systems require that the powder be transported to the consolidation head which is some distance from the point at which metering occurs; this leads to time lags between making an adjustment and achieving a new settled flow rate, so that the results are erratic. The systems cannot be used reliably for small flows. Also, after the systems have been adjusted to give a constant flow rate, it is not possible to quickly make small precise adjustments to this rate.
Experiments made using flow control valves which change the cross-sectional area of the supply conduits produced unpredictable results with slugging at the consolidation nozzle.
In accordance with the present invention, a regulator device for regulating the flow of particulate material comprises:
a first, inlet conduit having an outlet end,
a second, outlet conduit generally aligned with the first conduit and having an inlet end sufficiently close to the said outlet end of the first conduit as to be capable of receiving at least a portion of a particulate material flow exiting the outlet end,
a chamber surrounding the said inlet end of the second conduit for receiving particulate material which exits the outlet end of the first conduit and is not received into the inlet end of the second conduit, and
means for adjusting the area of the inlet end of the second conduit which is exposed to the said outlet end of the first conduit thereby to adjust the flow of particulate entering the second conduit.
The area of the inlet end of the second conduit which is exposed to the outlet end of the first conduit and which receives the material is varied by changing the lateral relationship of the adjacent ends of the two conduits. This includes the case where one of the conduits rotates relative to the other conduit and where the ends are non-circular or where the ends are eccentric to each other. Conveniently, the conduits have a common axis, and the inlet and outlet ends have non-circular cross-sections, so that relative rotation the between the adjacent conduit ends about their common axis alters the proportion of the second conduit inlet end which is exposed to flow of particulate material from the first conduit. The excess material, which goes into the chamber surrounding the inlet end of the second conduit, is passed through a by-pass tube and recycled.
The lateral relationship of the adjacent ends of the conduits can also be changed by shifting one conduit end along a path perpendicular to its axis relative to the other conduit end. This has the advantage over the rotational methods described above in it can include a no-flow condition where all the powder is diverted and recycled.
Preferably, the adjacent ends of the conduits are each in the form of a nozzle having a tip with an elongated cross-section with a major axis and a minor axis, the tips being close together, and the adjusting means causes relative rotation of one conduit relative to the other between a first angular position, in which the cross-sections of the nozzle have their major axes aligned so that the inlet nozzle of the second conduit can receive all, or at least most, of the particulate material exiting the outlet nozzle of the first conduit, and a second position in which one of the nozzles has been rotated about the axis of the second conduit through a substantial angle, up to about a right angle, relative to the other nozzle, whereby flow of particulate material into the inlet nozzle of the second conduit is reduced.
Preferably, the opposed, adjacent nozzles are both of similar elongated cross section, for example of elongated rectangular shape, and each have a major axis which is much larger than the minor axis, for example four, six or ten times larger. In this case, rotating the inlet end nozzle of the second conduit relative to the outlet nozzle of the first conduit by a relatively small amount, for example 10 or 20xc2x0, from the aligned condition, will cause a rapid reduction in the flow of material, which may be desirable when the invention is used for the laser consolidation process. When the nozzles are near the 90xc2x0 position, the adjustment of flow by rotating the nozzles becomes very fine.
Preferably, the first and second conduits are mounted in aligned first and second bores of a valve housing, and means are provided for adjusting the rotational relationship of the two nozzles about the common axis of the bores thereby to adjust the flow of particulate material between a maximum flow position where the cross-sections of the nozzles are in rotational alignment, and a minimum flow position at a substantial angle to the first position, usually about 90xc2x0. The means for adjusting may be a handle projecting laterally from the housing which is rotated by hand. Alternatively, the means for adjusting may be a motor controlled by a computer which also controls the operation of the consolidation process.
Preferably, the regulator is used with a carrier gas for the particulate material, as is conventional in powder delivery systems. When used with a carrier gas, the regulator of this invention has the advantages over many other systems in that firstly the flow of powder and of the gas remain in a constant proportion as the flow of powder is reduced, and in that secondly the speed and momentum of the carrier gas are not much affected by the adjustment to flow.
The regulator of this invention can be fairly small and compact, and can be made suitable for mounting close to or on the consolidation head which carries the laser and a tube delivering the powder and carrier gas to the workpiece being built or repaired. Accordingly, in accordance with another aspect of the invention, the regulator of this invention is carried by the consolidation head, or by a carrier which also supports the consolidation head, and is connected via a conduit to a conventional gas transport system which is situated close to a powder feed hopper. This allows for rapid and accurate adjustment of the powder feed rate as is required by the consolidation process.
The regulator of this invention does require an input flow rate which is constant. To achieve this, the regulator can be used in series with another, similar regulator on the input side, to give a yet further improvement in regulation.
In accordance with another aspect of the invention, a method for regulating the flow of particulate material comprises:
feeding the material and a carrier gas through a first conduit to exit at an outlet end thereof;
receiving a part of the material and gas into an inlet end of a second conduit which is generally aligned with the first conduit, the said ends being sufficiently adjacent for the second conduit to receive at least a portion of the material and gas exiting the first conduit; and
adjusting the lateral relationship of the adjacent ends of the conduits so that the amount of material flowing into the second conduit is adjusted, while recycling the material leaving the first conduit, and which does not flow into the second conduit, back to a supply source.